The present invention relates to a curable composition for polymer electrolyte as well as a polymer electrolyte produced therefrom and a polymer battery in which the same is used.
For a polymer electrolyte to be usable in lithium ion batteries or electrochemical devices, it is essential that it shows a high ionic conductivity in a wide temperature range, from low to high temperatures, and shows no crystallinity. However, any polymer electrolyte meeting such necessary performance characteristics requirements collectively has not yet been developed.
In the art, such organic solvent as propylene carbonate and ethyl methyl carbonate, for instance, are widely used in polymer electrolytes to be used in polymer batteries and the like. From the boiling point/vapor pressure viewpoint, however, they generally impose limitations on the use in a high temperature range of 70 to 90xc2x0 C.
Recently, polymer electrolytes, typically polyethylene oxide (hereinafter referred to as xe2x80x9cPEOxe2x80x9d), have been studied as means for improving the safety of such organic solvents. PEO forms complexes with salts of metals belonging to the group 1 or 2 of the periodic table of the elements, for example LiCF3SO3, LiClO4, NaCF3SO3 and LiI, to show relatively good levels of ionic conductivity in a temperature range not lower than room temperature and, further, shows good storage stability. However, the ionic conductivity of PEO is highly dependent on the temperature and, while it shows good ionic conductivity at 60xc2x0 C. or above, the ionic conductivity markedly lowers at temperatures not higher than 20xc2x0 C. Therefore, it is difficult to incorporate it in general-purpose products which may be used at low temperatures.
As a means of improving the ionic conductivity using low-molecular PEO, a method of introducing low-molecular PEO into side chains of a vinyl polymer has been reported by D. J. Banistar et al. in Polymer, 25, 1600 (1984). Although this high-molecular material forms complexes with lithium salts, the ionic conductivity at low temperatures is not satisfactory.
Further, materials derived from polysiloxanes by introduction of low-molecular PEO onto side chains thereof are described in Journal of Power Source, 20, 327 (1987), Japanese Kokai Publication Sho-63-136409 and Japanese Kokai Publication Hei-02-265927. They are, however, insufficient in ionic conductivity, are not noncrystalline, are not easy to synthesize, occur as liquids and are poor in workability or moldability, and are insufficient in mechanical strength. For these and other reasons, they have not been put to practical use.
A hydrosilylated crosslinked compound derived from a PEO side chain- and SiH group-containing polysiloxane and an olefin having polyethylene oxide in its main chain is described in Japanese Kokai Publication Hei-03-115359. However, the ionic conductivity thereof is considerably low, namely about 4.9xc3x9710xe2x88x926 Sxc2x7cmxe2x88x921, and this is not satisfactory.
It is an object of the present invention to provide a curable composition capable of giving a polymer electrolyte showing a high level of ionic conductivity and excellent in mechanical strength as well. Another object is to provide a polymer battery excellent in electrochemical characteristics.
The invention provides a curable composition for polymer electrolyte;
which comprises the following constituents (A) to (D) as an essential constituent:
(A) a SiH group-containing polysiloxane;
(B) a compound having at least one structure selected from the group consisting of a phenylene unit, a siloxy linkage, an Sixe2x80x94N bond, a carbonyl group, an amide linkage and an amino group and having two or more alkenyl groups;
(C) a hydrosilylation catalyst; and
(D) an electrolyte salt compound.
In a preferred embodiment of the invention, the curable composition for polymer electrolyte comprises the following constituents (A) to (D) as an essential constituent:
(A) a polysiloxane having a polyethylene oxide structure-containing group and/or a cyclic carbonate structure-containing group as a substituent on a silicon atom and having two or more SiH groups;
(B) a compound having at least one structure selected from the group consisting of a phenylene unit, a siloxy linkage, an Sixe2x80x94N bond, a carbonyl group, an amide linkage and an amino group and having two or more alkenyl groups;
(C) a hydrosilylation catalyst; and
(D) an electrolyte salt compound.
The invention also provides a polymer electrolyte obtained from the above curable composition for polymer electrolyte as well as a polymer battery having a structure such that the above polymer electrolyte is disposed between an anode and a cathode.
Constituent A
In the practice of the invention, any of those SiH group-containing polysiloxanes which are known in the art can be used as the constituent (A), without any limitation.
The constituent (A) polysiloxane preferably has a polyethylene oxide structure-containing group, a cyclic carbonate structure-containing group and/or a cyclic ether structure-containing group as a substituent on a silicon atom and further has two or more SiH groups. The one having a polyethylene oxide structure-containing group and/or a cyclic carbonate structure-containing group as a substituent on a silicon atom and further having two or more SiH groups is more preferred among others. In particular, from the high ionic conductivity viewpoint, the one having a polyethylene oxide structure-containing group and having two or more SiH groups is more preferred. In cases where the polymer electrolyte of the invention is used in combination with a carbonate, which is to serve as an electrolyte, the one having a polyethylene oxide structure-containing group and a cyclic carbonate structure-containing group and further having two or more SiH groups is still more preferred.
The polyethylene oxide structure-containing group so referred to herein is not particularly restricted but may be any of oxyethylene unit-containing univalent groups. The oxyethylene unit(s) may be bonded to a silicon atom either directly or via a bivalent organic group. The cyclic carbonate structure-containing group or cyclic ether structure-containing group is not particularly restricted but may be any of cyclic carbonate- or cyclic ether-containing univalent groups. The cyclic carbonate or cyclic ether may be bonded to a silicon atom either directly or via a bivalent organic group.
In cases where the constituent (A) polysiloxane has a polyethylene oxide structure-containing group as a substituent on a silicon atom, it is desirable, from the low crystallinity viewpoint, that 10 to 95% of all silicon atoms in the constituent (A) polysiloxane each has, as a substituent thereon, a polyethylene oxide structure-containing group with a degree of polymerization of the oxyethylene unit of 1 to 12 and it is more desirable that 40 to 90% of all silicon atoms in the constituent (A) polysiloxane each has, as a substituent thereon, a polyethylene oxide structure-containing group with a degree of polymerization of the oxyethylene unit of 1 to 12.
When the constituent (A) polysiloxane has a polyethylene oxide structure-containing group as a substituent on a silicon atom, the constituent (A) is preferably represented by the following structural formula: 
wherein m and n each is an integer of not less than 1, p is an integer of 1 to 12 and R represents a hydrogen atom or a hydrocarbon group containing 1 to 20 carbon atoms and, when n is not less than 2, the each R may be the same or different, provided that at least one of the R""s is a hydrogen atom; the arrangement of the m repeating units and n repeating units may be not in order.
When the constituent (A) is represented by the above formula, the polyethylene oxide introduction percentage (%, hereinafter referred to as G) defined below is preferably 10 to 95%, more preferably 40 to 90%.
G=[m/(m+n+2)]xc3x97100.
The values of m and n can be calculated with ease by determining the substituent content ratio by 1H NMR, for instance.
When the constituent (A) polysiloxane has a polyethylene oxide structure-containing group as a substituent on a silicon atom, the constituent (A) shows a high level of permittivity and is excellent in ability to dissolve and dissociate supporting electrolytes, since the polysiloxane has the polyethylene oxide structure on a side chain thereof. Further, since the main chain thereof has a siloxane structure, its glass transition temperature is low and this facilitates the transfer of ions. Such a polymer is highly stable at high temperatures. Therefore, the prevention of degradation at high temperatures and the high ionic conductivity occurrence at low temperatures, which have not been achieved with the prior art polymer electrolytes, can be accomplished in accordance with the present invention.
When the constituent (A) polysiloxane has a cyclic carbonate structure-containing group as a substituent on a silicon atom, the constituent (A) is preferably represented by the following structural formula: 
wherein m and n each is an integer of not less than 1 and R represents a hydrogen atom or a hydrocarbon group containing 1 to 20 carbon atoms and, when n is not less than 2, the each R may be the same or different, provided that at least one of the R""s is a hydrogen atom; the arrangement of the m repeating units and n repeating units may be not in order.
When the constituent (A) polysiloxane has a cyclic ether structure-containing group as a substituent on a silicon atom, the constituent (A) is preferably represented by the following structural formula: 
wherein m and n each is an integer of not less than 1 and R represents a hydrogen atom or a hydrocarbon group containing 1 to 20 carbon atoms and, when n is not less than 2, the each R may be the same or different, provided that at least one of the R""s is a hydrogen atom; the arrangement of the m repeating units and n repeating units may be not in order.
When the constituent (A) polysiloxane has a cyclic carbonate structure-containing group or a cyclic ether structure-containing group as a substituent on a silicon atom, too, the permittivity of the constituent (A) becomes high and the constituent (A) is excellent in ability to dissolve and dissociate supporting electrolytes. Since its main chain has a siloxane structure, its glass transition temperature is low, which facilitates the transfer of ions. Such a polymer is highly stable at high temperatures: Therefore, the prevention of degradation at high temperatures and the high ionic conductivity occurrence at low temperatures, which have not been achieved with the prior art polymer electrolytes, can be accomplished in accordance with the present invention.
Preferably, the constituent (A) polysiloxane is substantially free of any hydrolyzable silyl group (group capable of undergoing mutual binding and condensation to form a siloxane bond in the presence of water; specifically SiOH and SiOR groups (R being an alkyl group, an aryl group or the like). If the constituent (A) polysiloxane has a hydrolyzable silyl group, the hydrolyzable silyl group reacts with an SiH group to form hydrogen or an alcohol. As a result, the SiH group content decreases and no sufficient crosslinked structure is formed and/or the hydrogen or alcohol generation causes foaming in the curing product, with the result that it becomes difficult to obtain satisfactory membranes.
The constituent (A) polysiloxane preferably has a weight average molecular weight Mw (on the polystyrene equivalent basis) of 600 to 100,000, more preferably 2,000 to 100,000.
The constituent (A) may comprise one single species or a combination of two or more species.
The constituent (A) SiH group-containing polysiloxane can be synthesized, for example, by the process mentioned below. The method of preparing the constituent (A) is not limited to that process, however.
To a polyorganohydrogensiloxane are added, in a solvent, dropwise a hydrosilylation catalyst and olefin-terminated polyethylene oxide to thereby effect the hydrosilylation reaction and, after thorough stirring, the solvent is distilled off under reduced pressure, whereby a polysiloxane having a polyethylene oxide structure-containing group as a substituent on a silicon atom can be obtained.
The polysiloxane to be used in the above process preferably has a weight average molecular weight Mw (on the polystyrene equivalent basis) of 2,000 to 100,000.
The solvent to be used here is not particularly restricted but includes, as preferred species, toluene and the like.
The reaction temperature is not particularly restricted but the reaction is preferably carried out at room temperature to 100xc2x0 C.
The ratio between the olefin-terminated polyethylene oxide to be added and the SiH groups in the polysiloxane (olefinic group/SiH mole ratio) is preferably within the range of 0.10 to 0.95, more preferably 0.40 to 0.90, most preferably 0.50 to 0.85.
The hydrosilylation catalyst is not particularly restricted but includes, as preferred species, platinum compounds, rhodium compounds and ruthenium compounds. As examples, there may be mentioned platinum-vinylsiloxane and chloroplatinic acid.
This production process can be carried out batchwise, semibatchwise or continuously. The reaction vessel may be a continuous mixing tank reaction vessel, for instance. This process is preferably carried out batchwise or continuously.
For obtaining a polysiloxane having both a polyethylene oxide structure-containing group and a cyclic carbonate structure-containing group each as a substituent on a silicon atom, a terminal olefin-containing cyclic carbonate compound as well as the olefin-terminated polyethylene oxide are added.
Other polysiloxanes can be obtained in the same manner.
Constituent B
Any of those compounds known in the art which have at least one structure selected from the group consisting of a phenylene unit, a siloxy linkage, an Sixe2x80x94N bond, a carbonyl group, an amide linkage and an amino group and have two or more alkenyl groups can be used as the constituent (B), without any limitation. The constituent (B) preferably has a number average molecular weight Mn [GPC (polystyrene equivalent basis)] within the range of 80 to 1,000.
It is preferred that the constituent (B) is substantially free of any polyethylene oxide structure, in particular any polyalkylene oxide. Since the constituent (B) is a constituent for crosslinking the constituent (A) polysiloxane, the occurrence of such structure in the constituent (B) tends to decrease the ionic conductivity.
From the strength and moldability viewpoint, it is preferred that the constituent (B) have a low molecular weight, specifically of not more than 500, more preferably not more than 400. When the molecular weight is high, the ionic conductivity tends to lower.
As preferred examples of the constituent (B), there may be mentioned compounds having a phenylene unit and two or more alkenyl groups, compounds having a siloxy linkage and two or more alkenyl groups, compounds having an Sixe2x80x94N bond and two or more alkenyl groups, compounds having a carbonyl group and two or more alkenyl groups, compounds having an amide linkage and two or more alkenyl groups, compounds having an amino group and two or more alkenyl groups and compounds having a phenylene unit and a carbonyl group and two or more alkenyl groups.
Specifically, the constituent (B) includes bisphenol A diallyl ether, 2,2xe2x80x2-diallylbisphenol A, divinylbenzene, 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, 1,3-divinyl-1,1,3,3-tetramethyldisilazane, diallyl maleate, diallyl succinate, diallyl carbonate, diallyl dicarbonate, allyl-terminated acrylic polymers, 1,3-diallylurea, diallylamine and diallyl phthalate.
The constituent (B) may comprise one single species or a combination of two or more species.
Constituent C
In the practice of the present invention, any of those hydrosilylation catalysts which are known in the art may be used as the constituent (C), without any limitation.
Preferred as the constituent (C) is at least one compound selected from the group consisting of platinum compounds, ruthenium compounds and rhodium compounds. Platinum compounds are more preferred.
As preferred species of the constituent (C), there may be mentioned platinum-vinylsiloxane, chloroplatinic acid, Pt(COD)2 and the like.
The constituent (C) may comprise one single species or a combination of two or more species.
Constituent D
In the practice of the present invention, any of those electrolyte salt compounds which are known in the art may be used as the constituent (D), without any limitation.
Preferred as the constituent (D) are composed of: a cation selected from the group consisting of metal cations, ammonium ions, amidinium ions and guanidium ions and an anion selected from the group consisting of chloride ion, bromide ion, iodide ion, perchlorate ion, thiocyanate ion, tetrafluoroborate ion, nitrate ion, AsF6xe2x88x92, PF6xe2x88x92, stearylsulfonate ion, octylsulfonate ion, dodecylbenzenesulfonate ion, naphthalenesulfonate ion, dodecylnaphthalenesulfonate ion, R1SO3xe2x88x92, (R1SO2) (R2SO2)Nxe2x88x92 and (R1SO2) (R2SO2) (R3SO2)Cxe2x88x92 [in each formula, R1, R2 and R3 each representing an electron attracting group].
The electron-attracting groups represented by R1, R2 and R3 in R1SO3xe2x88x92, (R1SO2) (R2SO2) Nxe2x88x92 and (R1SO2) (R2SO2) (R3SO2) Cxe2x88x92 may be the same or different and each is preferably a perfluoroalkyl group containing 1 to 6 carbon atoms or a perfluoroaryl group.
The metal cation in the constituent (D) is preferably a cation of a metal selected from the group consisting of metals belonging to the group 1 or 2 of the periodic table and transition metals, in particular manganese, iron, cobalt, nickel, copper, zinc and silver. The lithium cation is particularly preferred.
Specifically, LiClO4, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2 or Li(C2F5SO2)2 is particularly preferred as the constituent (D).
The constituent (D) may comprise one single species or a combination of two or more species.
In the curable composition for polymer electrolyte according to the invention, the mole ratio (A)/(B) between the constitutent (A) and constituent (B) is preferably 0.01 to 5.0, more preferably 0.05 to 3.0. The constituent (C) hydrosilylation catalyst is preferably used in an amount of 0.000001 to 0.1 mole, more preferably 0.00001 to 0.01 mole, per mole of the double bond in the constituent (B).
The constituent (D) electrolyte salt compound is contained in the curable composition for polymer electrolyte preferably in an amount within the range of 0.01 to 10 millimoles, more preferably 0.10 to 5.0 millimoles, per gram of that composition.
The curable composition for polymer electrolyte according to the invention provides a sufficiently high level of ionic conductivity. However, when a higher level of ionic conductivity is required, an organic electrolyte may further be incorporated. As such organic electrolyte, there may be mentioned propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, xcex3-butyrolactone, 1,3-dioxolane, dimethoxyethane, tetrahydrofuran, dimethyl sulfoxide and polyethylene glycol dimethyl ether. Among them, carbonates are preferred. From the ion conductivity/strength compatibility viewpoint, the addition amount of the organic electrolyte is preferably 10 to 90% by weight, more preferably 30 to 70% by weight, based on the constituent (A).
In the curable composition for polymer electrolyte according to the invention, there may be incorporated a further polymer, another amphiphilic solvent and/or the like.
The curable composition for polymer electrolyte according to the invention, when crosslinked by the hydrosilylation reaction, forms a three-dimensional network structure. Therefore, the tasks unachievable with the prior art polymer electrolytes, namely fluidity prevention and improvements in mechanical strength and workability/moldability, can be accomplished in accordance with the present invention.
Method of polymer electrolyte production
The curable composition for polymer electrolyte can be produced by admixing the thus-obtained SiH group-containing polysiloxane (A) with the compound (B) having at least one structure selected from the group consisting of a phenylene unit, a siloxy linkage, an Sixe2x80x94N bond, a carbonyl group, an amide linkage and an amino group and have two or more alkenyl groups, the hydrosilylation catalyst (C) and the electrolyte salt compound (D), if necessary together with an organic electrolyte. For facilitating the molding thereof, an organic solvent may be added.
Upon heating this composition, the organic solvent, when contained therein, is evaporated and the curing reaction (hydrosilylation reaction) is allowed to proceed to give a polymer electrolyte. This polymer electrolyte is preferably caused to have a filmy shape.
The temperature in the heating step is not particularly restricted but preferably is within the range of room temperature to 150xc2x0 C., more preferably room temperature to 120xc2x0 C., most preferably 70 to 100xc2x0 C.
In the practice of the invention, the method of polymer electrolyte production is not particularly restricted. The kind of the reaction vessel is of no importance. For preventing side reactions from occurring, however, the production process is preferably carried out in a reaction vessel made of a nonreactive material.
A polymer electrolyte containing an organic electrolyte can be obtained by heating an organic electrolyte-containing curable composition or by heating an organic electrolyte-free curable composition and impregnating the thus-obtained curing product with the organic electrolyte. The organic electrolyte-containing polymer electrolyte generally occurs as a gel.
From the strength viewpoint, it is also preferable to produce a polymer electrolyte provided with a nonwoven fabric. This can be obtained by impregnating a nonwoven fabric with the curable composition for polymer electrolyte of the invention and heating the fabric to cause the hydrosilylation reaction to proceed.
When the curable composition for polymer electrolyte as disclosed herein is used, it is easy to obtain a large-area thin film-like polymer electrolyte having flexibility, which is one of the characteristic features of macromolecules.
Polymer battery production
Polymer batteries, in particular lithium-polymer batteries, can be constructed using the polymer electrolyte obtained in accordance with the invention. Thus, polymer batteries can be made by disposing the polymer electrolyte of the invention between an anode and a cathode.
As preferred anode materials for lithium-polymer batteries, there may be mentioned, among others, lithium-manganese double oxide, lithium cobaltate, vanadium pentoxide, polyacene, polypyrene, polyaniline, polyphenylene, polyphenylene sulfide, polyphenylene oxide, polypyrrole, polyfuran, polyazulene and, further, sulfur compounds. Such anode materials may also be used in the form impregnated with the curable composition of the invention in making anodes.
Preferred as cathode materials are, for example, metallic lithium, lithium-lead alloys, other lithium alloys, inorganic materials with lithium occluded therein, and carbonaceous materials with lithium occluded between graphite or carbon layers.
It is also conceivable to use the polymer electrolyte of the invention as a diaphragm for ion electrodes sensitive to cations such as alkali metal, copper, calcium and magnesium ions to thereby utilize the high ionic conductivity thereof.